1. Field of the Invention
The present invention relates to a thin organic composite film-coated steel sheet which has an excellent press-processability, a high spot-weldability and a high resistance to corrosion, and which is improved in susceptibility to cationic electrodeposition with particularly less formation of pinholes due to gas-evolution and an increase of throwing power, even in a bath after used for a longer period of time, thereby allowing the sheet to be used for the coated steel sheet as a material for use in production of automobiles, domestic electronic articles, buildings, containers, and the like.
2. Description of Related Art
Recently, there has been an increasing need for a less expensive zinc or zinc alloy-plated steel sheet having a higher rustproofing property for use in automobiles, domestic electronic articles, buildings, containers and the like. For this reason, intensive researches have been made to develop new plated steel products.
For example, Japanese Patent KOKAI (Laid-Open) No. Sho 61-23766 and Japanese Patent KOKOKU (Post-Exam. Publn.) No. Sho 62-20024 disclose a weldable steel sheet coated with a thin film of an electrically conductive paint containing metal particles, typically Zincrometal. However, these plated steels suffer practically from occurrence of galling with a die, and unacceptable appearance due to scratches caused by the friction between peeled coating films and a metal die during press-processing. Japanese Patent KOKOKU (Post-Exam. Publn.) No. Sho 55-51032, Japanese Patent KOKAI (Laid-Open) No. Sho 59-162278 and Japanese Patent KOKAI (Laid-Open) No. Sho 61-584 disclose that a high rustproofing property is achieved by coating the surface of the zinc or zinc alloy plated steel sheet with an aqueous dispersion containing chromic acid or chromium compounds. However, these also suffer practically from sweating and elution of chromium into aqueous treating solutions though they have a high rustproofing property. Japanese Patent KOKAI (Laid-Open) No. Sho 60-50181, and Japanese Patent KOKAI (Laid-Open) No. Sho 60-149786 disclose metal plates deposited with a thin coating film from a blend of an emulsion of a water-dispersible resin and an organic silicate complex (silica sol, silane coupling agent). These do not have any problem on the aforementioned chromium elution, but still suffer from a lower water resistance among physical properties of the coating due to the existence of alkali, ammonium ions and the like which are required for maintaining the silica in a colloidal form. In the paint containing fine silica in a solvent, its viscosity becomes too high to effect uniform coating owing to structural variation. Though it may be considered to disrupt the viscous structure by hydrogen bonding with alcohols, it cannot be applied to a system containing polyisocyanate compounds as curing agents. Moreover, there have been requirements for .circle.1 a higher resistance to corrosion, .circle.2 an enhancement of the bonding between a steel sheet and a coating film formed thereon (especially, a cationic electrodeposited film), and .circle.3 a lower baking temperature (140 to 170.degree. C.) in increasing the strength of processed steel sheet. As a process satisfying the need and solving the aforementioned problems, the present inventors have proposed an organic composite plated steel sheet which exhibits no elution of deleterious materials such as chromium and the like during surface treating by users, and which has a higher resistance to corrosion, a higher processability, and a higher susceptibility to electrodeposition due to a thin coating film baked at lower temperatures, as disclosed in Japanese patent Application No. Sho 62-168393. However, the process for producing such organic composite plated steel sheets has disadvantages that no inclusion of conductive materials such as metal particles in the coating films causes a reduction of the electric current through thicker film portions, and particularly a reduction of the depositing ability of the cationic electrodepositing bath after used for a longer period of time as occurring actually in the bath employed by automobile makers. To cope with the difficulties, a temporary expedient has been employed to provide micropores in the coating films rendering them more electrically conductive, or to provide a minimum deposition as a film thickness of 0.1 .mu.m.